The present invention relates to a method and apparatus for driving a raster scan video display device to display a plurality of grey levels. The invention is suitable for driving one bit-per-pixel devices in which the displays comprise a plurality of rows each consisting of a plurality of pixels. For example, the invention is particularly suitable for use in driving simple matrix liquid crystal displays.
Currently available simple matrix liquid crystal displays exhibit a high level of contrast between the bright and dark areas of the display. The high contrast ratio makes these displays suitable for supporting a number of grey levels, i.e. levels of brightness intermediate the brightness of a pixel when it is continuously illuminated and when it is not illuminated. These displays are designed with a standard control interface which enables each pixel in the display to be turned "on" i.e. illuminated, or "off" i.e. not illuminated, independently in every frame. Since the displays are inherently one bit-per-pixel devices, each pixel of the display may only be specified as either on or off in any one frame, and it is not possible to set the individual pixels to any intermediate grey levels. However, the liquid crystal material is slow to respond to changes in state (from on to off and vice versa). This property allows grey levels to be displayed by turning the pixels on and off in successive frames, the slow response of the liquid crystal material enabling the eye effectively to average the rapid state changes of the pixels so that the overall appearance of the display is that of a grey level. The perceived grey level of an area of the screen is dependent upon the proportion of pixels illuminated in that area in successive frames, although the quality of the grey level produced depends greatly upon the way the pattern of illuminated pixels changes with time.
A technique for generating grey levels on simple matrix liquid crystal displays is known in which complete rows of pixels are illuminated in each frame, and the rows which are illuminated are varied between successive frames. Grey levels of approximately 1/2, 1/3, 1/5, and 1/8 are achieved by lighting one row of pixels in every 2, 3, 5, and 8 rows respectively. In each case, the illuminated rows are varied between frames to avoid a completely static display. This technique, where complete rows of pixels are illuminated, gives rise to "striping effects" in the perceived grey level where bands of bright and dark appear to progress across the screen. This problem becomes more acute as the distance between illuminated rows in each frame increases. In addition, there is poor resolution for the grey levels around 50% full brightness.
A further example of a known method of producing grey levels on a liquid crystal display is provided in GB 2164776A (Canon). This discloses a driving system for a liquid crystal display panel in which the duration of the "on" or "off" state of each pixel in each frame is controlled in order to produce a graduated display.